On January 26, 1781, King George III reluctantly gave his assent to the Britannic Design, a bill that had been sent to him three days earlier after passage by Parliament. The King had been opposed to what he regarded as lenient treatment of the American rebels by the North ministry. After Lord North sent the original version of the Design to Parliament, the King used his friends there to wage a struggle against it.
Ever since coming to the throne in 1760, George III had been attempting to recover the powers of the monarchy which had been allowed to lapse by his grandfather and great-grandfather. The influence he wielded over Parliament contributed greatly to the worsening of relations between the American colonists and the British government, and ultimately led to the outbreak of the Rebellion. As Sobel notes, Lord North's decision to ignore the King's wishes in formulating policy for the colonies after the Rebellion resulted in his government, and those of his successors, being more independent of Royal control than at any time since the Commonwealth (when there was literally no Royal control of the government, since the monarchy had been overthrown).
Ever since the establishment of the C.N.A. in July 1782, January 26 has been celebrated as a patriotic holiday called Design Day.